Computer systems incorporating a microcomputer and an input-output unit connected together by a large number of conductors are well-known and are described, for example, in German patent publication 2 900 111, U.S. Pat. No. 4,204,256 and U.S. patent application no. 56,960 (filing data July 12, 1979) now U.S. Pat. No. 4,250,858.
The input-output unit serves to supply external information to the computer unit and to make computer commands and information available externally. In order to obtain the most effective utilization of the input-output circuit it is sought to have as few control connections between the computer unit and the input-output unit, while at the same time having available as many as possible different external connections and, also making it possible being able to fit specific input-output circuits to different computer units as simply as possible.
The known systems usually require the following connections between the input-output unit and the computer unit: a data bus (for 8 bit bytes, 8 conductors), an address bus (e.g., 4 conductors) a write command (MWR) connection, a read command (MRD) connection, a strobe or timing connection, a program interrupt connection (usually referred to simply as the "interrupt" connection) and a "clear" command connection for causing the memory of the input-output unit to be cleared. The selection of the particular circuit of the input-output unit by the computer unit is provided through the address bus that reaches or seizes the location to be selected in the input-output unit simultaneously with the transmission of a data byte.